1. Field of the Invention
The present invention generally relates to x-ray tomography apparatuses and, in particular, to those apparatuses which are adapted for CAT scan operations.
2. Statement of the Prior Art
CAT scans (computerized axial tomography) have been used for many years. Generally the system operates by taking multiple, cross-sectional, slice x-rays from different angles within a single plane passing through the body. The results are mathematically compiled to create a cross-sectional image of the body in that plane. To produce these x-rays in one form, an x-ray source and an array of detectors are placed on opposite sides of an annular yoke, which yoke is made to rotate within the selected plane and around the patient.
An important consideration in scanning has been the accurate and consistent alignment of the tomography components and the patient both throughout the rotation and over the course of many scans and patients. Misalignment or movement can negatively influence the data of an entire scan. In order to deal with this factor, manufacturers of CAT scan apparatuses have typically produced a very large and massive machine which includes a heavy yoke for mounting the tomography components. The handling of this weight requires additional mass in the remainder of the apparatus and typically a large apparatus.
One ramification of the extra size and mass of these machines has been the requirement of a patient handling apparatus, as the weight of the patient is typically much less than the weight of the yoke and other rotating components. Thus, a moveable patient table has been used to properly position the patient in the desired location relative to the fixed yoke, and this further contributes to the size and weight of the apparatus.
A further contibutor to the size and mass of these machines has been the problem of delivering electrical power for the x-ray source to the rotating apparatus. The two approaches primarily used have been electrical brushes, or slip rings, which constantly sweep during rotation and extended cables which limit the rotation of the yoke to approximately one revolution. Unfortunately, the brush approach creates a disruptive amount of electrical interference in the very sensitive output signals of the detectors. Some of this interference can be reduced by the use of shielding; however, the shielding must be extensive as the brushes rotate around the entire large circumference of the yoke. The shielding also adds bulk and weight. More accurate machines use the extended cables which limit rotation. Unfortunately, this approach requires much larger motors and produces greater system stresses and wear, because the yoke and all of the of the moving mass must be accelerated and decelerated quickly in order to adequately operate within the limited rotational range of the yoke.
The apparatus which results from these various requirements is large, heavy, expensive and one which is difficult to relocate. It requires a large amount of floor space and thus cannot be used in space limited environments. A further disadvantage caused by the size and weight of these apparatuses is the wear experienced in the moving parts thereof.
In another form of tomography apparatus known in the prior art, the x-ray source and an array of detectors are mounted on a "C" shaped frame which is typically cantilevered and manipulated over the patient. Although this apparatus can be manipulated over a wide range, it also requires large amounts of space and mass for the manipulation apparatus.